Goo on Crossover Bulbs

[TomCarlos]

Does anyone know what there would be so much buildup of the Goo on these bulbs? And what is it?

Thanks!!

[g1]

I would guess hot melt glue. Mainly to protect the bulbs from the vibration of the speaker.

[TomCarlos]

Thanks G1.... that makes sense.

[x-pro]

There's no place for hot glue. Just silicone. It holds temperature.

[Pedro Vecino]

Ampeg? Crate? If so, it's not exactly traditional silicone. It is a less flexible compound with less adhesion. It can be easily removed with the tip of a screwdriver.

[Jon Snell]

The technical name for it is Room Temperature Vulcaniser. Soft and squidgy. Comes off moderately easily.
Comes in many colours.
Black for gaskets, grey or yellow for electronics as it is self curing.

[g1]

Tom, was it a soft or hard substance? I just guessed based on looks and color.
I don't think these bulbs should be heating up under normal operation, although they might if the horn is being pushed too hard.

[TomCarlos]

G1 and others.....

My friend sent me this photo. So I will assume it is hard yet crusty. And I agree.... I doubt these things are lighting up. I will see if my bud can send me the ID for the board. Maybe I can post a schematic.

[x-pro]

No. This silicone is soft to the touch, not crunchy. It can be cut with a knife with a little effort.
I have changed such bulbs in system filters.

[Leo_Gnardo]

IIRC Enzo suggested hitting this sort of goo with freeze spray. Once chilled and hardened it's a bit easier to chip away.

I've seen these sort of lamps in Community crossovers. I'm sure they're used as a current limiter / fuse in others besides. Had to replace a couple some years ago. I found 4 amp 12V lamps to fit at a local auto parts store.

Story time: I've also seen speaker cabs set alight (pun intended??? maybe...) when smacked with serious high power. 15 March 1992 at the Augusta GA tennis stadium, in the middle of the BIG ROCK CONCERT a handful of cabs one side of the Community PA started smoking then blazing. Could have done without that, but I'm sure the limiter / fuse lamps saved the speakers. That's what they're supposed to do, right? Inspecting a roasted cab at the post show post mortem it was apparent the lamps were the source of ignition. Lesson: if you see bright light flashing from the interior of your PA cab, immediately dial down your volume. Don't press your luck. Might want to mount those lamps in such a way they can't set nearby materials on fire. Gobs of goo on the lamps don't sound like a great idea to me. Keep in mind, if the lamps I replaced were 4A 12W, when operated at spec that's 48 watts, with a lot of the energy dissipated as infrared in other words HEAT. Sometimes there are several lamps mounted closely together. You can see how the heat generated might set nearby flammable material ablaze.

[TomCarlos]

A follow up. This schematic looks awful close to the photo. The bulbs are part of the high frequency section of the crossover.

[mhuss]

Yep, power limiting for the HF driver(s). When these bulbs pass enough current to light up, their resistance goes way up.

[Helmholtz]

The bulbs are used as (over-) current limiters using the PTC characteristic of the filament (and eventually act as fuses).
Principle is the same as with an LBL before an amp.

Tweeters/ HF drivers are endangered when there's HF acoustic feedback or when the PA amp clips producing strong HF harmonics.
First saw this used around 1980.